The present invention relates to an image heating apparatus such as a thermal fixing device mounted in an image forming apparatus, for example, a copying machine or a printer. In particular, it relates to an image heating apparatus having a rotational member, which contacts a recording medium on which an image is borne, and a regulating member for regulating the movement of the rotational member in the direction parallel to its generatrix direction.
In the image forming portion of an image forming apparatus which employs an image formation process, for example, an electrophotographic process and an electrostatic recording process, an unfixed toner image in accordance with the image formation information for an intended image is directly formed on, or transferred onto, a recording medium (transfer medium, printing paper, photosensitive paper, electrostatic recording paper, and the like). This unfixed toner image on the recording medium is thermally fixed to the recording medium by a thermal fixing apparatus; in other words, the unfixed image is turned into a permanent image by a thermal fixing apparatus. As for a thermal fixing apparatus, a heating apparatus employing the so-called roller heating method is widely used. A roller heating type thermal fixing apparatus comprises a heat roller (fixing roller) as a heating member, and a pressure roller as a pressure applying member. The two rollers are rotated while kept pressed against each other, by the application of a predetermined amount of pressure. In operation, while a recording medium bearing an unfixed toner image is conveyed through the nip formed between the heat and pressure rollers, the unfixed image is fixed to the recording medium; it is turned into a permanent image.
In recent years, due to the concerns regarding xe2x80x9cquick startxe2x80x9d and energy conservation, a film heating type heating apparatus has been put to practical use, which has been disclosed in, for example; Japanese Laid-open Patent Applications 63-313182, 2-157878, 4-44075, and 4-204980.
A film heating type heating apparatus comprises: a heating member, for example, a ceramic heater; a pressure roller as a pressure applying member; and a heat resistant resin film (which hereinafter will be referred to as fixation film). The pressure roller is kept pressed against the heating member, forming a compression nip (which hereinafter will be referred to as fixation nip). In operation, a recording medium bearing an unfixed toner image is introduced into the fixation nip, and conveyed through the fixation nip, together with the fixation film, with the recording medium pinched between the fixation film and pressure roller, while the heating member, or the ceramic heater, conducts heat to the recording medium through the fixation film. As a result, the unfixed image on the recording medium is fixed to the surface of the recording medium, by the heat and compressive force in the fixation nip.
A film heating type heating apparatus is capable of becoming ready to heat by the time a recording medium reaches the heating apparatus, even if power begins to be supplied to the heater after the reception of a print signal by the image forming apparatus. In other words, while a film heating type heating apparatus is kept on standby, the heater of a film heating type heating apparatus does not need to be supplied with power. Thus, from the standpoint of energy conservation, a film heating type heating apparatus is an excellent thermal fixing apparatus in that it does not waste energy.
Replacing the resin fixation film, as a heating member, of a film heating type heating apparatus, with a thin metallic sleeve, the base layer of which is formed of a metal superior in thermal conductivity to the resin fixation film, improves the fixation performance of the fixing apparatus, enabling the fixing apparatus to satisfactorily cope with the increase in the speed of an image forming apparatus.
The fixation film, or sleeve, tends to shift in the thrust direction. Thus, a film heating type heating apparatus in accordance with the prior arts is provided with a regulating member, such as a sleeve end flange, for regulating this shifting of the fixation film in the thrust direction, more specifically, for catching the fixation film by one of its edges. With the provision of this type of structural arrangement, the fixation film which is rotated while being kept in contact with the flat surface of the heater, is repeatedly flexed while one of its edges is remaining in contact with the corresponding sleeve end flange. Eventually, the edge portions of the metallic film develop cracks. This phenomenon is more severe in the case that a metallic sleeve (fixation film formed of thin sheet of metallic material) comprising a base layer formed of metallic material superior in thermal conductivity to resin fixation film, is used as a heating member.
More specifically, as the edge of a fixation film is placed in contact with a sleeve end flange due to the shifting of the fixation film, it is subjected to such force that acts in the direction to increase the diameter of the edge portion of the fixation film, that is, in a manner to expand the edge portion of the fixation film like the end of a trumpet. This force is greater in the case of a metallic fixation film, or sleeve, for the following reason. That is, a metallic sleeve is supported from inside by a flange, being therefore not regulated in terms of its expansion in its radius direction. Thus, its edge portion is more likely to be deformed in the form of the end portion of a trumpet by the above described force. Obviously, the edge of a metallic sleeve rubs against a sleeve end regulation flange as does any fixation film. Therefore, a metallic fixation film (sleeve) is more likely to develop cracks along the edges.
In addition, if the metallic base layer of a metallic sleeve is excessively thick, it takes a long time for the temperature of the heater to be raised to a level at which satisfactory fixation is possible, after a printing operation is initiated at room temperature; it takes too long for the heater temperature to rise to a predetermined level; the waiting time is long.
In other words, if the metallic base layer of a metallic sleeve is excessively thick, it is difficult to reduce the length of the time (first print time) from the reception of a print signal to the completion of the first print.
Further, if a metallic sleeve is thick, it is inferior in elastic deformation which is essential for the metallic sleeve to contact the heater surface, without leaving any gap between the two. Therefore, it is difficult for the heat from the heater to efficiently conduct to the metallic sleeve.
Further, the thicker the metallic sleeve, the higher the pressure which must be applied to ideally place the metallic sleeve in contact with the heater surface. In other words, in the case of a relatively thick metallic sleeve, it must be forcefully bent. Therefore, it does not stand long use; it breaks down due to fatigue.
As will be evident from the above description, a structural arrangement which is capable of making a thin metallic sleeve in accordance with the prior arts, satisfactory in terms of thermal and mechanical strength, has not been found.
As for a method for solving the above described problem, it is possible to provide a metallic sleeve with edge reinforcement members. However, a metallic sleeve must be able to elastically deform in order to conform to the flat heater surface, and it has been virtually impossible to form an edge reinforcement member which does not interfere with the elastic deformation of a metallic sleeve, which is necessary for the metallic sleeve to conform to the flat heater surface. More specifically, when an edge reinforcement member is secured to the edge of a metallic sleeve by gluing or the like method, it must be able to flex with the metallic sleeve. However, in the case of a film heating type heating apparatus in accordance with the prior arts, in which the metallic sleeve is in contact with the flat heater surface from one end of the heater to the other in terms of the lengthwise direction of the heater, if a part of the edge reinforcement member is on the inward surface of the metallic sleeve, it comes into contact (interferes) with the heater surface, resulting in the frictional damage to the edge reinforcement member and/or heater surface.
It is also possible to make such a structural arrangement that the portions of the heater surface correspondent in position to the edge portions of a metallic sleeve, that is, the portions fitted with an edge portion reinforcement member, are kept away from the inward surface of the metallic sleeve, in order to prevent the above described problem. However, it is extremely difficult to process a heater, in particular, a thin ceramic heater with a fixed thermal capacity in order to give it such a structure. Also, it is extremely costly. In other words, the idea is not practical. Further, from the standpoint of the reliability of current flow, it is extremely difficult to provide the electrically conductive portion of a heating member, or the like, on a substrate, with areas which are not level with the other portion of the conductive portion.
As will be evident from the above, it is difficult to realize a thermal fixing apparatus which employs a metallic sleeve, is satisfactory in fixing performance, and also is durable. So far, a means for realizing such a thermal fixing apparatus has not been provided.
The present invention was made in consideration of the above described problem. Thus, its primary object is to provide an image heating apparatus superior in the durability of its rotational member.
Another object of the present invention is to provide an image heating apparatus which is compact in the measurement in terms of the direction parallel to its generatrix direction.
According to an aspect of the present invention, an image heating apparatus comprises:
a rotational member which contacts an image bearing recording medium;
a regulating member for regulating the movement of said rotational member in the direction parallel to the direction of the generatrix of the rotational member;
wherein said regulating member is secured so that it does not rotate, and is provided with a surface which faces peripheral surface of the lengthwise end portion of said rotational member.
According to another aspect of the present invention, an image heating apparatus comprises:
a rotational member which contacts an image bearing recording medium;
a first regulating member for regulating the movement of said rotational member in the direction parallel to the direction of the generatrix of the rotational member, said first regulating member being enabled to rotate following the rotation of said rotational member; and
a second regulating member for regulating the movement of the said first regulating member in the direction parallel to the generatrix direction, said second regulating member being nonrotationally secured;
wherein said first regulating member is in the form of a cylindrical ring, and said second regulating member is shaped like a xe2x80x9ccylindricalxe2x80x9d pillar with a missing segment.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.